Kinetics of Phosphorus Release from Struvite and Struchar Prepared from Domestic Sewage Sludge

Municipal wastewater is a rich source of phosphorus. Due to the limited non-renewable resources of phosphate in the world, the recovery of phosphorus from wastewater in the form of struvite or magnesium ammonium phosphate (MgNH4PO4.6H2O) as slow release fertilizer has been considered worldwide. In this study, phosphorus removal from sewage sludge with digestion-absorption process with H2SO4 (0.2 N) and the molar ratio of magnesium/ ammonium/ phosphorus in 2:1:1 at pH=8.5. The struvite and struchar precipitation were performed in different ratio of grape biochar to struvite including 0:100, 25:75, 50:50 and 75:25 and some of their physicochemical properties were identified. Kinetic Phosphorus release of samples were investigated at 24 hr, 3, 5, 10, 14, 28, 42, 56 and 84 days in distilled water, citric acid (20g/L) and HCl (0.5 M). The release of phosphorus, magnesium and nitrogen in the samples increased with increasing contact time. The results showed that the phosphorus-cumulative-release of composites in water and 20 gram per liter critic acid good fitted to the first-order kinetic model (R2 =0.97-0.99), whereas it followed the Elovich model (R2 =0.98-0.99) in 0.5 mole per liter hydrochloric acid. In addition, the analysis of samples showed that the concentration of heavy metals studied in this study, increased in the presence of biochar but were less than the allowable concentration in fertilizers. In XRF analysis, P2O5 has the highest composition of the obtained sediments. By increasing percentage of biochar the release of phosphorus, nitrogen and magnesium increased in water, citric acid 20 g/L and HCl 0.5 M and the maximum solubility of phosphorus was obtained in the ratio of 75:25 biochar to struvite. Therefore, the use of waste grape pruning biochar in the struvite deposition process increases phosphorus removal, solubility and phosphorus release rate.